Magnetic sensor circuits typically include a Hall device that outputs an output voltage proportional to the intensity of a magnetic field, an amplifier that amplifies the output voltage of the Hall device, and a comparator that compares an output voltage of the amplifier with a predetermined reference voltage to output a comparison result, and the magnetic sensor circuit outputs a binary signal (which is either high level or low level at a time) according to whether or not the intensity of the magnetic field at the site in which the magnetic field sensor is placed is higher than a given standard.
To obtain an accurate comparison result that reflects the intensity of the magnetic field, it is necessary to reduce an offset signal component contained in the signal outputted from the amplifier to reduce the variation of this signal. Main factors that produce the offset signal component here are the offset signal component contained in the output voltage of the Hall device (hereinafter, “device offset voltage”) and the offset signal component present at the input terminal of the amplifier (hereinafter, “input offset voltage”). The device offset voltage is generated chiefly by stress or the like that the Hall device proper receives from its package. On the other hand, the input offset voltage is generated chiefly by variations or the like in the characteristics of the devices that form the input stage of the amplifier.
A magnetic field sensor that is less affected by such offset voltages is disclosed in Patent Document 1 listed below. The magnetic field sensor incorporates a Hall device, which is, like the Hall device 1 shown in FIG. 14, typically formed as a plate having a shape that is geometrically equivalent with respect to four terminals A, C, B, and D. Here, a geometrically equivalent shape denotes one, like the shape of the Hall device 1 shown in FIG. 14, whose shape in one orientation is identical with its shape in a 90 degrees rotated orientation (rotated such that diagonal A-C now lies where diagonal BD lay before). In this Hall device 1, between the voltage that appears between terminals B and D when a power supply voltage is applied between terminals A and C and the voltage that appears between terminals A and C when the power supply voltage is applied between terminals B and D, effective signal components contained respectively in them—the components commensurate with the intensity of the magnetic field—are in-phase, whereas the device offset voltages contained respectively in them are in opposite phases.
First, in a first period, through a switch circuit 2, the supply voltage is applied between terminals A and C of the Hall device 1, and the voltage between terminals B and D is fed to a voltage amplifier 3. Thus, the voltage amplifier 3 outputs a voltage V1 proportional to the sum of the voltage between terminals B and D and the input offset voltage of the voltage amplifier 3. Moreover, in this first period, a switch 5 is closed, so that a capacitor 4 is charged up to the voltage V1.
Subsequently, in a second period, through the switch circuit 2, the supply voltage is applied between terminals B and D of the Hall device 1, and the voltage between terminals C and A is fed to the voltage amplifier 3 with the polarity opposite to that in the first period. Thus, the voltage amplifier 3 outputs a voltage V2 proportional to the sum of the voltage between terminals C and A and the input offset voltage of the voltage amplifier 3.
Here, irrespective of the polarity of the input voltage, the influence of the input offset voltage remains the same as in the first period. Accordingly, the voltage V2 from the voltage amplifier 3 is proportional to the sum of the voltage between terminals C and A—a voltage of the polarity opposite to that in the first period—and the input offset voltage.
Moreover, in this second period, the switch 5 is open, so that an inverting output terminal 3a and a non-inverting output terminal 3b of the voltage amplifier 3 and the capacitor 4 are connected in series between output terminals 6 and 7. Here, the charge voltage of the capacitor 4 remains unchanged from, and is thus held equal to, the output voltage V1 of the voltage amplifier 3 in the first period. The voltage V between the output terminals 6 and 7 (the output voltage of the magnetic field sensor) equals the sum of the voltage V2 at the non-inverting output terminal 3b of the voltage amplifier 3 relative to that at its inverting output terminal 3a and the voltage—V1 at one end 4a of the capacitor 4 relative to that at its other end 4b, that is, the voltage V2 minus the voltage V1. In this way, the influence of the input offset voltage is cancelled out, and thus the magnetic field sensor yields, as its output voltage, the voltage V free from it.
Also conventionally known is a magnetic field sensor that not only is less affected by the device offset voltage but also is less affected by the input offset voltage arising in the amplifier, as disclosed in Patent Document 2 listed below. This magnetic field sensor is composed of a Hall device, a switch circuit, a voltage-current converter-amplifier, a capacitor as a memory device, a switch, and a resistor.
As another example of conventional technologies related to the above description, a magnetic sensor circuit that not only is less affected by the device offset voltage but also is less affected by the input offset voltage arising in the amplifier is disclosed and proposed in Patent Document 3 listed below filed by the applicant of this application.    Patent Document 1: Japanese Patent Registered No. 3315397, Specification    Patent Document 2: JP-A-H08-201491    Patent Document 3: International Publication WO 2006/085503, Pamphlet